What is the difference between a "main account" and a "side account"?

Only one of your Neopets accounts can be designated as a main account.

Users should use their main account to earn Neopoints, play Flash games, manage their shops, and do the site's dailies. It is against the rules for side accounts to earn Neopoints, play Flash games, etc. You may transfer Neopoints from your main account to a side account in order to purchase food and other items for the Neopets on the side account. Users who earn Neopoints on their side accounts are in violation of the site rules and could have their accounts frozen.